Pursuant to the development of electronics in recent years, semiconductor products are being utilized in various fields. Since semiconductor products are generally manufactured from a semiconductor wafer, the quality management of semiconductor wafers is important to achieve enhanced performance of the semiconductor products. As one index of evaluating the quality of a semiconductor wafer, there is the life (lifetime) of a carrier in the semiconductor. Particularly in recent years, photovoltaic (PV) cells are attracting attention as a clean energy source. With these photovoltaic cells, it is important for the carriers (electrons and holes) that are generated by irradiation of light to reach the electrode without any recombining midway in order to achieve high photoelectric conversion efficiency. Thus, it is also important to evaluate the carrier lifetime in a PV semiconductor wafer. Base on the evaluation of the carrier lifetime, it is possible to improve the yield of photovoltaic cells by sorting, during the production process, the PV semiconductor wafers that are unable to achieve the required specification (spec). Consequently, it is also possible to achieve cost reduction.
As one method of measuring the carrier lifetime, a microwave photoconductive decay method (μ-PCD method) is known. The microwave photoconductive decay method is a method which generates excess carriers by irradiating light onto a semiconductor (semiconductor sample, measured sample) to be measured, and detects the process of the excess carriers becoming recombined and extinct in the carrier lifetime that is determined based on the physicality of the semiconductor sample based on the time change of reflectance or the time change of transmittance of the microwave. Since the generation of excess carriers increases the conductivity of the semiconductor, with the microwave that is irradiated onto the site (portion, region) of the semiconductor in which excess carriers were generated via photoexcitation, its reflectance or transmittance changes in accordance with the density of the excess carriers. This microwave photoconductive decay method is used for measuring the carrier lifetime by utilizing the foregoing phenomenon.
Generally speaking, crystalline imperfection exists on the surface of the semiconductor wafer, and so-called surface recombination where carriers recombine on the semiconductor surface will consequently arise. Thus, the measurement results of the carrier lifetime not only include the carrier lifetime (bulk carrier lifetime) based on the internal recombination of the semiconductor wafer, but also include the carrier lifetime (surface carrier lifetime) based on the foregoing surface recombination. The bulk carrier lifetime is important in the quality management of the semiconductor wafers, and the carrier lifetime based on surface recombination results in a measurement error. Thus, normally, the semiconductor sample is subject to heat treatment prior to being measured and an oxide film is formed on the semiconductor sample surface so as to inhibit the generation of the surface recombination, or the semiconductor sample is dipped in a solution containing, for example, iodine prior to being measured so that so-called dangling bonds or the like that cause surface recombination are passivated. This kind of pretreatment that is performed in advance is troublesome and time-consuming, and the semiconductor wafer may be subject to performance loss due to the heat treatment or the semiconductor wafer may be contaminated due as a result of being dipped in a chemical. Thus, a method of more easily measuring the bulk carrier lifetime in the semiconductor wafer is being demanded, and, for instance, thereafter the technologies disclosed in Patent Document 1 to Patent Document 3, and the technology disclosed in Non-Patent Document 1.
The carrier lifetime measuring method disclosed in Patent Document 1 is a carrier lifetime measuring method based on the photoexcitation method of injecting excess carriers, via photoexcitation, to the vicinity of the surface of the semiconductor substrate in a state of thermal equilibrium, and detecting and measuring the temporal change in the amount of reflection of the microwave upon viewing the decay process of the excess carrier concentration as the change in conductance, wherein an insulating film is provided to the surface of the semiconductor substrate prior to measuring the carrier lifetime, forming a charge layer thereon, and using a corona discharge for accumulating positive or negative charge on the insulating film surface provided to the semiconductor substrate in order to form the charge layer.
With the carrier lifetime measuring method disclosed in Patent Document 1, since the charge layer, which is generated by a corona discharge, below the insulating layer will easily discharge, surface recombination will occur during the measurement of the lifetime of the semiconductor carrier, and it is thereby difficult to accurately measure the lifetime of the semiconductor carrier.
Thus, the semiconductor carrier lifetime measuring apparatus disclosed in Patent Document 2 is a semiconductor carrier lifetime measuring apparatus which measures the semiconductor carrier lifetime by measuring the change in the reflected or transmitted wave of a predetermined measurement wave that was irradiated onto the semiconductor when pulsed light is irradiated onto the semiconductor, and comprises a waveguide for guiding the measurement wave to the surface of the semiconductor, and a first electrode which is provided to a portion of the waveguide that is adjacent to the semiconductor or in the vicinity thereof, and in which a predetermined voltage is applied and causes a corona discharge at least during the measurement of the change in the reflected wave or the transmitted wave of the measurement wave.
Moreover, with the lifetime measuring method disclosed in Non-Patent Document 1, at least two types of pulsed light having different wavelengths and different lengths of penetration are irradiated onto a semiconductor, photoexcited carriers are thereby generated in the semiconductor, and a temporal relative change in the reflected wave or the transmitted wave which decreases based on the recombination of the photoexcited carriers and the difference thereof are thereafter detected. According to the lifetime measuring method disclosed in Non-Patent Document 1, it is possible to analytically separate the surface carrier extinction and the bulk carrier extinction regardless of the surface combination velocity of the wafer surface. Non-Patent Document 1 describes that, consequently, the bulk carrier lifetime can be extracted.
Meanwhile, the semiconductor carrier measuring apparatus disclosed in foregoing Patent Document 2 is advantageous in that it is not necessary to perform any pretreatment process in advance such as heating or oxide film formation, and that it is possible to maintain the charged state in the measurement wave irradiated portion of the semiconductor during the measurement thereof, but a measurement error will occur if the charged state changes due to a discharge during the measurement. Moreover, much time is required for stabilizing the charged state, and the semiconductor carrier measuring apparatus disclosed in Patent Document 2 is not suitable for measuring the carrier lifetime and sorting the semiconductor wafers during the production process; that is, in the production line. Due to the foregoing reasons, there is room for improvement in the semiconductor carrier measuring apparatus disclosed in Patent Document 2.
Moreover, with the surface recombination velocity as S and the diffusion coefficient as D, the value that is obtained from the measurement results of the lifetime measuring method disclosed in foregoing Non-Patent Document 1 is S/D, and the lifetime measuring method disclosed in Non-Patent Document 1 obtains the surface recombination velocity S by assuming D=30 cm2/s, and thereby obtains the carrier lifetime. Nevertheless, when the carrier concentrations of electrodes and holes are n and p, respectively, and the diffusion coefficients of electrodes and holes are Dn and Dp, respectively, the actual diffusion coefficient is given as (n+p)/(n/Dp+p/Dn), and this is dependent on the carrier concentration or conduction, and is not constant. In addition, when the surface recombination velocity S is relatively large such as when pretreatment is not performed in advance, the measured (observed) carrier lifetime will be small in comparison to the bulk carrier lifetime, and its change will also be small, thereby causing the measurement error to increase.
Meanwhile, the method of measuring the semiconductor wafer characteristics disclosed in Patent Document 3 is a method which irradiates a light beam or an electron beam onto one surface and/or another surface of a semiconductor wafer, and detects a conductive time change of the semiconductor wafer based on at least two types of different spatial distributions caused by the difference in the carrier excitation conditions of the excess carriers that were instantaneously excited by the irradiation of the light beam or the electron beam so as to separately measure the surface recombination velocity of one surface of the semiconductor wafer and the surface recombination velocity and the bulk lifetime of the other surface recombination velocity, respectively.
As described above, the methods disclosed in Patent Document 3 and Non-Patent Document 1 are methods that measure the carrier lifetime by using the difference between the respective measurement results that are obtained based on mutually different conditions. Thus, when the difference between the respective measurement results is small, the number of significant figures in the difference between the respective measurement results will be small, and, consequently, it is difficult to accurately measure the carrier lifetime.
Patent Document 1: Japanese Patent Application Publication No. H7-240450    Patent Document 2: Japanese Patent Application Publication No. 2004-006756    Patent Document 3: Japanese Patent Application Publication No. S57-054338
Non-Patent Document 1: J. Appl. Phys. Vol. 69, (9), 6495 (1991)